The described invention addresses the low temperature deposition of titania and also, optionally, a process designed to achieve/retain a degree of photo-catalytic activity. This process seeks to, in part, bridge the significant technological “gap” between current AP thermal CVD and vacuum plasma CVD for the deposition. The invention describes a route to achieving the low substrate temperatures associated with plasma CVD, whilst avoiding the cost and process design constraints of a vacuum system. The invention also allows for much faster titania growth rates to be achieved, than normally possible with vacuum plasma CVD.
Although atmospheric pressure glow discharge plasmas (APGDP) have been known for some time, the application of such plasmas has been largely limited to surface treatment e.g. pre-treatment of plastics prior to printing or second stage coating.
In recent years a number of literature reports, and more recently patents, have covered the area of APGDP, their generation, and applications. Patents such as (U.S. Pat. Nos. 5,938,854 and 6,221,268) discuss application to surface treatment. A small number of literature reports cover the issue of using APGD plasmas for producing coatings on a surface. Primarily these are considering deposition of “plasma polymerised” films i.e. films which have a significant organic content or show characteristics which would not normally be considered as inorganic. (e.g. Goosens, Dekempeneer et al, Surface and Coatings Techn, 2001, and DE 19955880). A few patents deal with deposition of inorganic type films (e.g. U.S. Pat. No. 6,235,647) however the materials considered, and the approaches suggested are not optimised for industrial exploitation. To date no industrial application of APGDP activated CVD is known to us. We are also not aware of any reports of titania deposition via a APGD (Atmospheric Pressure Glow Discharge) CVD (Chemical Vapor Deposition) approach.
The invention described herein addresses these limitations, and defines a process or a method particularly compatible with the establishment of an industrially viable process for the deposition of functional titania coatings.
According to the invention, the method for depositing titania, or titania-containing as thin films on a substrate, the method comprising the steps of:                using an atmospheric pressure glow discharge plasma as a major source of reaction to improve film properties and film growth rates,        heating the substrate at a temperature below 250° C., preferably below 100° C.,        a reactive titania CVD precursor is introduced into a gas flowing through a coating region which has been pre-vaporised into the introduced gas flow.        
The method particularly addresses the need for high titania growth rates at lower temperatures than normally employed in APCVD processes. The process also identifies the importance of controlling plasma conditions and gas phase concentrations, to achieve target compositional, physical and functional properties.
In order to achieve target low temperature operation and target process characteristics the plasma type and operation details needs to be carefully selected. Many different types of plasmas exist, however a glow discharge plasma is particularly advantageous as it can be operated as a non-thermal plasma. A range of power sources and settings can generate such plasmas, however we have found that the use of low frequency AC plasmas give appropriate performance.
In such a case, where an appropriate plasma is used, the thermal temperature of the plasma is much lower than the electronic temperature. The preferred frequency range for this type of plasma at atmospheric pressure, is different from that normally used for vacuum plasma generation. This can be understood in terms of the generation and trapping of sufficient plasma species, within the plasma-coating zone, which will be moderated by the much higher gas molecule densities at AP. For example, diffusion rates, active species lifetimes and charge build up will all differ markedly with increased pressure operation. The frequency range below 100 KHz is typically proposed, and a number of reports use frequencies around 20 KHz or below. The optimum frequency will depend a number of factors including; reactor design, materials used, plasma gases chosen, additive concentrations, voltage and power levels employed.
The gases employed to support the GD plasma are normally selected from helium, argon and nitrogen (or mixtures thereof) although it is possible to introduce additional gases as minority components to achieve particular plasma characteristics (e.g. oxidising properties).
To achieve good quality optical and mechanical properties in the films of titania grown using this approach, and also to achieve photo-active titania, we have found it necessary to carefully control the plasma and chemical reactions occurring. An example of this is with water vapour levels in the reaction chamber to avoid unwanted reactions. Careful control of the oxidising source (typically oxygen gas but alternative oxygen containing species can be used e.g. organic oxygen containing species) is necessary to achieve optimum performance in terms of film properties.